This invention relates to a small motor having a permanent magnet rotor, in particular a two-pole permanent-magnet rotor, and a laminated stator, in particular a U-shaped stator, having limbs whose end portions form magnetic poles for the rotor, said limbs carrying exciter coils, in particular alternately energizable coils.
Small motors having permanent-magnet rotors feature a simple construction and, as a consequence, low manufacturing costs. As synchronous motors they are used particularly in the field of small domestic appliances. Such a small motor which is operated as a synchronous motor has been described in the magazine "Feinwerktechnik und Me.beta.technik" 87 (1979) 4, pages 163 to 169.
In the case of position-dependent electronic control of these motors, they will behave in the same way as d.c. commutator motors. The maximum torque of such motors is limited by their size. Neither the maximum field to be generated nor the coil losses allow the motor torque to be increased for given dimensions. Therefore, the motor speed must be increased if it is required to increase their output power.
For the electronic control of these motors the winding is divided into two halves arranged on iron shanks, switching transistors alternately driving the coil halves so as to allow a reversal of the field direction in the air gap (U.S. Pat. No. 3,333,172). As a result of the leakage inductance switching voltages arise when switching from one winding to the other. By means of a special arrangement the voltage surges of these switching voltages can be limited to values which are safe for the switching transistors. However, apart from the required material and the associated costs, this method also gives rise to losses.
The use of bifilar windings enables the leakage inductance to be reduced so far that said special arrangement can be dispensed with. U.S. Pat. No. 4,500,824 describes such a bifilar winding.
However, in the case where position-controlled permanent-magnet motors are used the bifflar winding arrangement leads to winding capacitances, which are even stepped up by the required wiring. At higher speeds the winding then comes within the range of the resonant frequency. This leads to a resonance step-up of the winding resistance and, ultimately, to a torque reduction. As a result of this effect the motor torque decreases as the speed increases.